Could Iran be building nuclear weapons? A scientific perspective

Tuesday, Mar 6, 2012, 03:22 AM | Source: The Conversation

Martin Sevior

Iran is constructing nuclear power stations; that much is clear. AAP

There is much concern that Iran is in the process of developing nuclear weapons. Such a development, we’re told, could induce Israel to launch a unilateral military strike with all types of unpredictable consequences.

Now Iran, of course, is a signatory to the Nuclear Non-Proliferation Treaty – unlike many other Middle East nations – and thus far the International Atomic Energy Agency (IAEA) has found no direct proof of nuclear weapons development in Iran. I don’t know whether Iran is developing nuclear weapons – or, if it is, why.

On the other hand, I can provide a bit of background on why the IAEA and many countries have come to be so concerned about Iran’s nuclear ambitions.

Iran’s first nuclear power plant, located outside the southwest city of Bushehr, was opened last year. It has already begun contributing electricity to the domestic power grid. Construction of a second power plant is underway in Darkhovin, north of Bushehr, and the country is dotted with nuclear research facilities, most prominently the Tehran Nuclear Research Center.

Of particular concern to the IAEA, though, are the uranium enrichment facilities located in Natanz and Fordow, both south of Tehran. To understand why enrichment facilities cause consternation in the international community, we have to understand the process of nuclear fuel enrichment.

I’ll start with some physics and chemistry. Chemical elements found in nature are distinguished from each other by the number of protons in their atomic nuclei. Every atom of each particular element has the same number of protons in its nucleus. But it’s not that simple: most chemical elements actually consist of a collection of different nuclear isotopes.

Isotopes of the same chemical element have the same number of protons in their nuclei, but different numbers of neutrons.

Different isotopes of hydrogen: Hydrogen-1 (with no neutrons), Hydrogen-2 (with one neutron), and Hydrogen-3 (with two neutrons). Wikimedia Commons

We can specify which isotope we’re talking about by identifying the combined number of protons and neutrons in the isotope’s nucleus. So, for example, naturally occurring potassium is made up of the isotopes Potassium-39, Potassium-40 and Potassium-41, with relative abundances of 93.26%, 0.01% and 6.73% respectively.

These numbers mean that 93.26% of naturally occurring potassium is composed of Potassium-39, and so on. All three potassium isotopes have almost exactly the same chemical properties, but their nuclei are completely different.

Why is this important? Because different isotopes of the same element can have very different properties. Unlike other potassium isotopes, Potassium-40 is radioactive. Potassium is an essential ingredient of all living organisms, and the nuclear radiation from the Potassium-40 within our bodies is responsible for about one quarter of our natural background radiation dose.

This brings us back to nuclear energy. Naturally occurring uranium consists of 99.3% Uranium-238 (U-238) and 0.7% Uranium-235 (U-235). Of the two, only U-235 undergoes nuclear fission – the splitting of atoms to generate massive amounts of energy – with low-energy neutrons.

(While U-238 will fission when bombarded with high-energy neutrons, not enough of these are emitted from the fission of other uranium nuclei to sustain a nuclear chain reaction.) As a result, most nuclear power plants need uranium fuel to be “enriched” in U-235.

This means increasing the relative concentration of U-235 in the uranium to 3.5%–5% relative to U-238, as opposed to 0.7%. Nuclear weapons, on the other hand, need U-235 to be concentrated to a much higher level – 80% or greater. Low-enrichment nuclear fuel cannot be made to explode like a nuclear weapon.

A model of the Uranium-235 isotope. Its nucleus contains 92 protons and 143 neutrons. Wikimedia Commons

To accomplish this enrichment, then, one must find a way to concentrate U-235 relative to U-238. This a very difficult task because one cannot use chemical processes to distinguish the two isotopes. The nuclear industry has now settled on centrifugal enrichment technology as the most economical method of separating U-235 and U-238.

In this process, uranium-hexaflouride – a processed, gaseous form of uranium – is spun extremely rapidly in a metal cylinder (the centrifuge). Since U-235 is slightly lighter than U-238, it tends to collect at the centre of the cylinder, where it is skimmed off.

The output of one centrifuge is fed into another, each one slightly enhancing the ratio of U-235 to U-238. The process is continued until the desired enrichment is obtained.

Monitoring the level of enrichment is crucial, both for the operator of the program and for outside observers such as the IAEA. Luckily, because of the different nuclear properties of U-235 and U-238, the enrichment level can very easily be measured.

U-235 is about ten-times more radioactive than U-238, and the pattern of gamma-rays from U-235 is very different from that of U-238. The combination of these two characteristics makes it easy to determine the relative concentrations of the two isotopes.

The IAEA does this with equipment placed outside containers holding the enriched uranium, the input uranium and the leftover tails from the process.

Containers of fresh high-enrichment uranium from a Chilean reactor. NNSANews

I find reports that Iran is enriching fuel to 20% – as opposed to the 5% required for electricity production – very worrying. Although uranium enriched to 20% will not make an effective nuclear weapon, it could be a sign they’re testing their procedures to make weapons-grade uranium.

On the other hand, some research reactors used to make medical radioisotopes require 20%-enriched uranium. This is the reason given by Iran for its production of higher-enrichment uranium.

Nevertheless, any plant capable of enriching uranium in sufficient quantities to make nuclear fuel can be configured to enrich that uranium to 80%. One simply feeds it though the sequence of centrifuges until the desired concentration is reached.

Because achieving 80% enrichment is the most complex and difficult part of manufacturing nuclear weapons, undeclared enrichment facilities represent the strongest technological indication of a nuclear weapons program – which is why they are monitored so closely by the IAEA.

Iran has the world’s second largest natural gas reserves, enough to supply the country’s domestic electricity needs for centuries. Furthermore, it is relatively easy for a government to buy nuclear fuel (albeit with conditions, such as being required to permit snap inspections of all nuclear facilities). In my opinion, it is not necessary for Iran to have built their own enrichment plants.

Nevertheless, now that the facilities have been built, it is easy enough for the IAEA (if given access) to determine the level of enrichment of the nuclear fuel being produced, and to make sure this matches the amount of natural uranium fed into the plant. This way they can detect whether any uranium has been diverted into other, undisclosed programs.

It is therefore vital, above all, that the IAEA inspectors continue to be allowed access to Iran’s nuclear facilities.

The Conversation

Martin Sevior's Ph.D. was made in the field of nuclear physics, although he now works in experimental particle physics. He has investigated nuclear power and the issues surrounding it but has not received funding to do so. He maintains the website

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